Fig. 3.6. Kiang (E. kiang) (picture provided by Zoological Society of San Diego).
Zebras
The best-known feature of the African zebras is probably their extensive, distinctive striped coat patterns. Despite this common characteristic, the zebra group is genetically quite diverse. As can be seen from Table 3.1, they are typically regarded as belonging to three species groups (E. grevyi, E. quagga and E. zebra), and have chromosome numbers ranging from 32 to 46.
Grevy’s zebra (E. grevyi; Fig. 3.7) is characterized as a single species. The species is not very numerous and is classified as endangered. Several subspecies have been described for E. quagga (Fig. 3.8). E. q. burchellii has the common name of Burchell’s zebra, common zebra or plains zebra. Other subspecies of E. quagga have been recognized as E. q. boehmi (Grant’s zebra), E. q. zambiensis, E. q. borensis (maneless zebra), E. q. chapmani (Chapman’s zebra), E. q. crawshayi (Crawshay’s zebra), and E. q. selousi (Selousi’s zebra). This group is denoted as of “least concern,” meaning that the populations appear to be stable.
Fig. 3.7. Grevy’s zebra (E.grevyi) (picture provided by Zoological Society of San Diego).
Fig. 3.8. Buchell’s zebra (E. quagga) (picture provided by Zoological Society of San Diego).
E. zebra (Fig. 3.9) is considered to be vulnerable and occurs in two populations: the Hartman’s mountain zebra (E. z. hartmannae) and the Cape Mountain zebra (E. z. zebra).
Fig. 3.9. Hartmann’s zebra (E. zebra hartmannae) (picture provided by Zoological Society of San Diego).
Zebra species occupy distinctive ranges with limited overlap, especially in modern time. Chromosome painting has been used to identify the differences in genome organization among the different species (Trifonov et al., 2008). For more information, see Hack and Lorenzen (2008), Moehlman et al. (2008a), and Novellie (2008).
Species Hybrids
Despite extensive chromosomal differences, the various equine species generally can be successfully crossed to produce viable progeny. In the late 1800s, a series of experiments was conducted to construct hybrids between zebras and horses and these were described in a book called The Penycuik Experiments (Ewart, 1899; now available as a digitized book). Such hybrids show that, despite extensive chromosomal and genomic changes, the overall genomic content is similar within the genus Equus and viable offspring can be produced. However, the hybrids are usually sterile, probably due to the differences in chromosome arrangements and failure to produce viable gametes following meiosis. During meiosis in hybrids, the chromosomes of the two parent species pair undergo recombination of DNA between chromosomes. However, because the genes are organized on different chromosomes, the resulting sperm and eggs have unbalanced numbers of genes and are non-viable. Even simple chromosome rearrangements that are sometimes observed in horses can result in unbalanced gametes and reduce fertility. With so many chromosome differences among Equus species, hybrids of distantly related equids are sterile. Only hybrids between closely related species, such as between the horses or among the hemiones, produce fertile offspring and, even then, hybrids are likely to have reduced fertility.
Rare fertility in mules and hinnies
Historically, the hybrid cross between the domestic horse and donkey species was agriculturally highly important. The offspring of a female horse and a male donkey is called a mule; the reciprocal cross between a female donkey and a male horse produces a hybrid called a hinny. Both hybrids have 63 chromosomes; 32 from the horse parent and 31 from the donkey parent. The ovaries of most female horse/donkey hybrids are usually non-functional (atrophic) and males do not produce sperm (they are azoospermic). The occurrence of fertile mules is very rare because of the failure to produce viable gametes. Some rare exceptions have been documented. Ryder et al. (1985) used karyotyping and blood typing to confirm the parentage of a jack (male) mule foal by a Welsh pony stallion out of a molly (female) mule. Karyotyping studies of fertile female mules and hinnies in China have also been reported (Rong et al., 1988). The explanation of documented fertile mules has been suggested to be a consequence of hemiclonal transmission, a phenomenon in which the mother passes along the chromosomes from the mother but not the father. In other words, these viable offspring are not true species hybrids. In these rare and well-documented cases, the offspring inherited only the domestic horse chromosomes and were genetically horses insofar as their chromosomes were concerned. Hemiclonal transmission has been observed in some amphibians but, other than these exceptional cases, is unknown in mammals. Because it is so rare, the mechanism for hemiclonal transmission in equids is a mystery.
Feral Species